1. Technical Field
The invention relates generally to a cleaning and etching process for wafers in semiconductor fabrication. More particularly, to methods and apparatus for preventing metal corrosion in an aqueous bath used in wet etching processes.
2. Background Art
Back end of the line (BEOL) structures, created on wafers, are subjected to an etching process to remove surface layers that are no longer needed after a particular step in the semiconductor fabrication process. The etching usually occurs in the presence of a plasma which results in residual particles/ions adhering to the surface of the wafers. During a wet clean rinse of these BEOL structures, corrosion of metal deposits or fillings, like copper or copper alloys, in lines and vias occurs, particularly for structures connected to n-type, p-type devices or structures connected down to the substrate.
When the wafers are immersed in a bath/solution to rinse off residual particles from reactive ion etching, the residual particles, which may be organic or inorganic materials, are released into the solution of the bath. These particles and the wafer carry a charge and a build-up of these particles and charges create an electrochemical effect transforming the bath into a galvanic cell. The transformed bath, as a galvanic cell has a high inherent affinity for positive ions. The metal deposits, exposed through a dielectric layer on the wafer, become a source for satisfying high affinity for positive ions. The tendency for metal deposits/fillings to dissolve is proportional to the electro-negativity of the solution. When wafers are immersed in the bath in batches, the depletion of metal deposits become more intense. The depletion of metals, for example, copper in the lines and vias of the BEOL structures compromises the yield of semiconductors devices. This is particularly evident for low-k dielectric materials after reactive ion etching. When wafers are cleaned in batches in the bath, the galvanic effect is accentuated and copper lined features are depleted at a greater rate.
In order to minimize the corrosive or galvanic effect during the wet clean rinse, ionic solutions or solutions of organic compounds, like that of benzotriazole (BTA) in deionized water, may be added to the bath. However this approach has its limitations in that the amount of BTA to be added will need to be varied according to the size and age of the bath as well as the batch sizes of wafers being rinsed. The type of wafers to be wet cleaned is also another factor to consider in varying the concentration of BTA added to the bath since different types of wafer have different requirements. For example, a different circuit design may require a different pattern density of the metal layer and changing the concentration of BTA in a single bath may be effective in neutralizing the galvanic effect for one BEOL structure but not for another of a different circuit layout design. It would not be efficient to have different bath compositions each time a different type of semiconductor product requires wet cleaning. Altering the concentration of the bath for accommodating multiple products also leads to difficulties with disposal and control.
In view of the foregoing, there is a need in the art for a solution to the problems of the related art.